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Appendix A: Additional experiments

To validate the formation of carbon dioxide, we performed further experiments using the HCO isotopologue of formaldehyde: the results are presented below. The top panel in Fig. A.1 shows three RAIR spectra recorded after irradiation of 2 ± 0.5 ML of HCO with increasing doses of oxygen atoms (0, 1.4, and 3.8 ML, respectively from a to c spectra) on oxidised HOPG at 55 K. Three main features are visible: at 1502 and 1697 cm^-1 due to HCO, ¹³CH₂ scissoring and ¹³CO stretch mode, respectively; at 2285 cm^-1 due to the antisymmetric stretch of ¹³CO₂. Moreover, a very weak band due to O₃ is present at 1042 cm^-1 (not shown). Each IR band varies in intensity from curve to curve. In particular, the HCO bands decrease from spectrum a to spectrum c, and the reverse occurs to the bands of ¹³CO₂. This indicates that HCO is consumed and ¹³CO₂ is formed subsequently to O irradiation. The infrared assignments are confirmed by TPD results shown in the bottom panel of Fig. A.1. These TPD traces were obtained after deposition of 2 ± 0.5 ML of ML of O atoms on oxidised HOPG held at 60 K. Newly formed ¹³CO₂ desorbs between 70 and 90 K and is detected via mass 45. Unreacted HCO molecules desorb between 85 and 140 K and are detected via two fragments: ⁺HCO (mass 31) and ⁺H¹³CO (mass 30).

	Fig. A.1 Top panel: three RAIR spectra obtained after deposition of (a) 2 ± 0.5 ML of HCO; (b) 2 ± 0.5 ML of ML of O atoms; (c) 2 ± 0.5 ML of ML of O atoms on oxidised HOPG held at 60 K. Bottom panel: 13CO2 (via mass 45) and (via masses 31 and 30) TPD traces obtained after deposition of 2 ± 0.5 ML of ML of O atoms on oxidised HOPG.
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	Fig. C.1 Optimized geometries: pre-reactive complex (PRC), Transition State 1 (TS1), and Transition State 2 (TS2), from the left to the right panels, respectively.
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Appendix B: Rate equations

The model used to fit our experimental data is very similar to the one described in Minissale (2014) and Minissale et al. (2013b; 2014). It is composed of five differential equations, one for each of the species considered: H₂CO molecules, deposited on the surface; O atoms, coming exclusively from the beam; O₃ and CO₂, formed only on the surface; and finally O₂, coming both from the beam and formed on the surface. Each differential equation is composed of different terms:

• positive terms indicate a contribution to the increase in the surface density of the species, i.e., a molecule (or atom) arrives from the gas phase or is formed on the surface;

• negative terms indicate a decrease in the surface density of the species, i.e., a molecule (or atom) is consumed on the surface or desorbs and returns to the gas phase.

Likewise, the terms involving the ER and LH mechanisms are independent of one another, thus we are able to determine the amount of a species formed (or consumed) via the ER or the LH mechanism. For the sake of simplicity, the differential equations were divided into two sets, accounting for the two formation mechanism separately. Below is the list of equations governing the CO₂ formation by the ER mechanism: O, O₂, O₃, H₂CO, and CO₂ are the surface densities (expressed in fractions of ML) of the species, τ is the dissociated fraction of O₂ defined in Sect. 2, φ_O2off is the flux (0.003 cm^-2 s^-1) of O₂, ϵ is the evaporation probability – due to chemical desorption − of O₂ formed on the surface (Dulieu et al. 2013), and are the reaction probability of H₂CO+O via ER and the desorption probabilities of O and O₂, respectively; ν = 10¹² is the trial frequency for attempting a new event. Finally, simple calculations show that 2τφ_O2off and (1 − τ)φ_O2off are the O and O₂ flux, respectively, when the discharge is on. Similarly, as for CO₂ formation by the LH mechanism, we have where are the thermal diffusion probability of O atoms and the reaction probability of H₂CO+O via LH, respectively. The activation barrier E_a is derived inverting the normalized Arrhenius equation: (B.16)with either r_aER or r_aLH according to the mechanism at play. Here (see Minissale et al. 2013b for more details).

Appendix C: Theoretical calculations

The geometries (Fig. C.1) and energies (Tables C.1 and C.2) were optimized either using DFT with the hybrid meta exchange-correlation functional M06-2X or using classical MP2 method, both methods coupled to the Dunning’s correlation (Dunning 1989) consistent polarized valence triple zeta basis set cc-pVTZ. The highly nonlocal M06-2X functional developed by Truhlar et al. (Zhao & Truhlar 2008) is relatively well suited to structures and energetics calculations of the transition states even though this method slightly underestimates barrier energy. We then also calculate the barrier height using full optimized MP2

method, as well as single point CCSD(T) with MP2 geometries in some cases. The unrestricted Hartree-Fock (UHF) formulation has been used since it is a convenient way to describe open-shell and bond-breaking processes. Its use is justified in our study by the fact that we did not observe any significant spin contamination for all the stationary points explored, the quantum average value ⟨S2⟩ of the square of the total spin operator remaining close to 2.00, i.e. the characteristic value for a triplet state. Full geometry optimization was performed throughout. We checked carefully that all the saddle points found are correctly connected to two minima and are characterized by the existence of only one negative eigenvalue of the Hessian matrix corresponding to an imaginary frequency in the normal mode analysis.

© ESO, 2015